1. Field of the Invention
The present invention relates to a developer bearing member and a process for producing the developer bearing member as well as a development apparatus and a development method using the same.
2. Description of the Related Art
Many electrophotographic methods are conventionally known. In general, a photoconductive substance is used to form an electric latent image on an electrostatic latent image bearing member (photosensitive member) by various units, and develops the latent image with a developer (toner) for visualization. Furthermore, the toner image is transferred to a transfer material such as paper as needed, and then the transferred image is fixed on the transfer material by heat, pressure, or the like to produce a copy.
Recently, printers and copiers have been required to have main bodies made smaller, and inevitably containers to store toner also have been required to be made smaller. Under these circumstances, a toner that has particles made nearly spherical is used as a toner that enables many sheets to be printed in a small amount. In addition, a toner whose particles are made uniformly spherical is less in local charge difference and the charge quantity is very uniform. For this reason, such a toner is also advantageous in terms of producing high-quality images. Therefore, making toner particles spherical as well as making particle sizes smaller and making particles finer, has come to be prevailing.
However, a toner having a small particle size has a large surface area per unit mass, and so its high surface charge is apt to become large during the course of development. In addition, a toner whose particles are made spherical has a smoother surface than conventional ground toner and its particles easily encapsulate a magnetic material, so its charge quantity tends to be too large.
Especially, as a developer bearing member rotates repeatedly, the charge quantity of the toner carried by the developer bearing member becomes too large because of contact with the developer bearing member, and in some cases, the toner is strongly attracted to the surface of the developer bearing member and immobilized on the surface. In this case, the toner is not transferred from the developer bearing member to a latent image on a photosensitive drum, in other words, a charge-up phenomenon occurs. This charge-up phenomenon is liable to occurs at low humidity. The occurrence of the charge-up phenomenon makes it difficult for the toner located at the upper portion of the toner layer carried by the developer bearing member to be charged, resulting in a decrease in the amount of toner for development. As a result, problems are raised in that lines on an electrophotographic image become thinner and the image density of a solid image is reduced. In addition, it is difficult for the toner that is not suitably charged because of the charge-up to be held in place on the developer bearing member, and in some cases, the toner flows out on the surface of the developer bearing member. In this case, defects in the form of blotches due to the toner flowing out, i.e., a blotch phenomenon appears on an electrophotographic image.
Moreover, a state in which a toner layer is formed is different between an image portion (toner-consuming portion) and a non-image portion, and in some cases, a charge state comes to be different. For example, when a location at which a solid image having a high image density has been developed once on the developer bearing member comes to a development location at the next rotation of the developer bearing member and a half-tone image is developed, what is called a sleeve ghost phenomenon is liable to occur in which traces of the solid image appear on the half-tone image.
As a method for controlling the charge quantity imparted to toner by a developer bearing member, a technique has been conventionally used in which the surface chargeability of the developer bearing member is changed. Japanese Patent Application Laid-Open No. H02-105181 and Japanese Patent Application Laid-Open No. H03-036570 disclose a method in which a developer bearing member is used having a coating layer in which an electrically conductive fine powder such as crystalline graphite and carbon is dispersed in a resin to improve the electrical conducting properties of the developer bearing member and to thereby prevent excess charging.
In addition, Japanese Patent Application Laid-Open No. H07-114216, Japanese Patent Application Laid-Open No. H10-293454 (U.S. Pat. No. 5,998,008), Japanese Patent Application Laid-Open No. H05-289413, and Japanese Patent Application Laid-Open No. H11-072969 disclose that in order to impart excellent negative chargeability to negatively chargeable toner, a positively chargeable charge control agent (a charge control agent whose frictional charge with a negatively chargeable toner is positive) is included in the surface layer of a developer bearing member.
Specifically, as such charge control agents, Japanese Patent Application Laid-Open No. H10-293454 discloses a nitrogen-containing heterocyclic compound, Japanese Patent Application Laid-Open No. H07-114216 discloses a quaternary ammonium salt compound, and Japanese Patent Application Laid-Open No. H05-289413 and Japanese Patent Application Laid-Open No. H11-072969 disclose a quaternary phosphonium salt compound, or nigrosine and modified products with metal salts of fatty acids.
In addition, Japanese Patent Application Laid-Open No. H03-196165 and Japanese Patent Application Laid-Open No. H05-216280 disclose that in order to impart excellent positive chargeability to a positively chargeable toner, a negatively chargeable charge control agent (a charge control agent whose charge due to friction with a positively chargeable toner is negative) is included in the surface layer of a developer bearing member. As such charge control agents, Japanese Patent Application Laid-Open No. H03-196165 and Japanese Patent Application Laid-Open No. H05-216280 disclose a fluorine-containing compound, or metal complexes such as a monoazo metal complex and an acetylacetone metal complex. However, the techniques disclosed by the above prior art rather increase the charge-up for toners that are easily charged.
On the other hand, in order to suppress the charge-up of toners that are easily charged, the use of a developer bearing member including a surface layer containing a charge control agent having the same polarity as the toner charge polarity is effective in some cases. That is, the use of a negative-polarity charge control agent for a resin coating layer of a developer bearing member can suppress the charge-up for toners that are easily negatively charged and is effective in preferably controlling the quantity of charge. However, if a control agent that is incompatible with resin, such as a negatively chargeable nigrosine dye and an azo-type iron complex, is dispersed, when a resin layer is formed, the control agent is unevenly distributed in the resin layer in some cases. In this case, it is difficult to uniformly charge the toner. In addition, the wear of the developer bearing member due to heavy-use is liable to cause loss of the charge control agent, with the result that the charge controlling effect is lowered and charging properties become poor, thereby producing image defects in some cases. In addition, if a fluorine-containing charge control agent is used, because of its high polarity, the control agent tends to be arranged on the surface of the coating layer. In this case, when the developer bearing member is worn due to heavy-use, the abundance ratio of the charge control agent is reduced, with the result that the charge controlling effect is lowered and charging stability to the toner becomes insufficient, thereby tending to produce image defects such as degraded image quality.
In addition, Japanese Patent Application Laid-Open No. 2002-040797 discloses a developer bearing member having a surface layer formed by using a coating material containing a quaternary ammonium salt and a phenolic resin. This developer bearing member can suppress the charge-up of a developer and stably impart an appropriate charge to the developer.